A spark plug is a part for generating a spark to burn a fuel in an engine. In order to generate the spark, a central electrode and a ground electrode is separated from each other by a predetermined gap. In order to improve an efficiency of combustion in engine, an electrode tip is coupled with the central electrode and the ground electrode.
In general, the central electrode and the ground electrode are made of nickel (Ni). The electrode tip coupled with the electrode is made of a platinum-group noble metal such as iridium (Ir) or an alloy thereof. The electrode tip is fixed to the central electrode and the ground electrode by welding. As an example of the welding method, there are laser welding, electron beam welding, resistance welding, and the like.
Recently, the laser welding has been actively developed as a method of coupling the electrode tip with the central electrode. The laser welding method has advantages of a small input heat amount, a thermal deformation, and availability for precision welding, but it has disadvantage that the method is sensitive to welding process parameters. In the laser welding method, a high density energy of the laser beam is concentrated on a connected portion between the central electrode or ground electrode and the noble metal tip. Therefore, Ni that is the main component of the electrode, and the iridium alloy are melted, so that a intermediate melting layer having an intermediate coefficient of linear expansion can be formed, and thus, a thermal stress can be alleviated.
According to a research, if an energy of the laser beam is lower, a dissolving temperature at the time of welding is lowered. Therefore, only the nickel is melted, but the thermal stress cannot be alleviated. This is because a difference of melting points of the nickel (about 1,450° C.) and the iridium (2,450° C.) is very great. Moreover, a boiling point (about 2,700° C.) of the nickel approaches the melting point of the iridium. If the energy of the laser beam is too high, a temperature of the dissolved portion becomes high, so that the nickel is evaporated, and thus, defects occur. Particularly, since the laser beam used for the laser welding is a Gaussian beam, a central portion thereof causes excessive input heat. Therefore, there is a problem of poor quality of welding such as element deficiency caused from the evaporation.
For these reasons, much research needs to be made in order to securely fix the electrode tip to the ground electrode or the central electrode by using the laser welding.